1. Field of the Invention
The present invention generally relates to a method of molding resin on a substrate, and particularly relates to a method of molding resin on a thin-film resin substrate that works well for high-frequency characteristics and a high-frequency module.
2. Description of the Related Art
There is a rapid decrease in size, thickness and weight of electronic equipment such as portable mobile communication terminals. The portable mobile terminals are installed with high-frequency modules, such as power amplifiers and high-frequency circuit boards.
Thus, it is necessary to reduce size, thickness and weight of the high-frequency modules to further reduce the size, thickness and weight of such portable mobile terminals. For this purpose, it has been proposed to select a thin-film resin substrate that can achieve further reduction of size, thickness and weight and can improve the high-frequency characteristics.
However, due to its reduced thickness, the thin-film resin substrates are often deformed during a manufacturing process of the high-frequency module. Accordingly, there is a need for a resin molding method which can restrict the deformation of the thin-film resin substrate during the manufacturing process.
FIGS. 1A and 1B are diagrams showing an example of a high-frequency module 1 of the related art. FIG. 1A is a plan view showing general structure of the high-frequency module 1. FIG. 1B is a cross-sectional view showing general construction of the high-frequency module 1. Such high-frequency module 1 may be used, for example, as a power amplifier of a portable mobile terminal. It is desirable to further reduce the size, thickness and weight of the high-frequency module 1.
Generally, the high-frequency module 1 includes a high-frequency circuit board 2, a high-frequency active chip 3, chip components 4, and a resin package 5. The high-frequency circuit board 2 includes a base material 15 of ceramics, glass-ceramics, or glass-epoxy. On a first (upper) surface of the base material 15, high-frequency circuit interconnections 6 and 7, direct-current (DC) circuit interconnections 8 and 9, and pad portions 12 to 14 are provided in a predetermined pattern. On a second (lower) surface of the base material 15, a ground layer 18 and land portions 19 are provided.
The high-frequency circuit board 2 of the above structure is provided with an opening 16 formed at a predetermined position of the base material 15. The high-frequency active chip 3 is mounted in the opening 16. Also, the high-frequency active chip 3 and each of the interconnections 6 to 9 are electrically connected by wires 17.
Also, a plurality of chip components 4 are mounted on the high-frequency circuit board 2. Each chip component 4 is joined to the interconnections 6 to 9 by means of a conductive material. Further, the pad portions 12 to 14 are electrically connected to the ground layer 18 formed on the second (lower) surface of the base material by means of via holes 20 formed through the base material 15.
A high-frequency input terminal RFIn, a high-frequency output terminal RFout and bias terminals 10 and 11 are provided at predetermined end portions of respective interconnections 6 to 9. The terminals RFIn, RFout, 10 and 11, respectively, are electrically connected to land portions 19 serving as external connection terminals by means of the via holes 20 formed through the base material 15.
When the high-frequency module 1 is mounted on the mounting board, the land portions 19 will be electrically connected to the mounting board. Also, a first (upper) surface of the high-frequency circuit board 2 is sealed, for example, using a metal cap (not shown).
However, if the base material 15 of the high-frequency circuit board 2 is made of ceramics, the cost of the high-frequency module will be increased since a ceramics material is more expensive than a resin material. If the base material 15 of the high-frequency circuit board 2 is made of materials such as ceramics, glass-ceramics or glass-epoxy, it is difficult to reduce the thickness of the base material 15 below 100 μm. Thus, this is contrary to the aim of reducing the thickness of the high-frequency module 1.
Also, it is difficult to implement a machining process on the prior art material used for the base material 15 with high accuracy. This is particularly problematic when forming the via holes 20. With the high-frequency module 1 for processing high-frequency signals, it is preferable to reduce impedance. However, since it is difficult to optionally select the thickness of the base material 15 and the diameter of the via hole 20, the impedance could not be reduced with the high-frequency module 1 of the related art. Accordingly, with the high-frequency module 1 of the related art, it is not possible to avoid any degradation of the characteristics due to the via holes 20.
Also, with the high-frequency module 1, it is desired to achieve a broader band with the same signal line width. In order to achieve broader band with the same signal line width, it is necessary to reduce the thickness of the base material 15 comprising the high-frequency circuit board 2 and to reduce the relative permittivity.
However, since the base material 1 has comparatively great thickness and relative permittivity, it is difficult to achieve broader band using the same signal line width. Therefore, with the circuit layout in a millimeter wave region using ceramics having high relative permittivity, the width of a 50Ω signal line becomes extremely small and thus becomes difficult to form such signal line.
In order to solve the problems described above, the inventor has proposed a high-frequency module 30A as shown in FIGS. 2 to 6 (JP 11-310159 A1).
Generally, the high-frequency module 30A includes a high-frequency circuit board 32A, a high-frequency active chip 33, chip components 34, and a resin package 35. The characteristic feature of the high-frequency circuit board 32A is that it includes a base material 45 of polyimide.
On a first (upper) surface of the base material 45, high-frequency circuit interconnections 36A and 37A (microstrip lines, coplanar lines etc.), direct-current (DC) circuit interconnections 38A and 39A, and pad portions 42 to 44 are formed in a predetermined pattern. The high-frequency circuit interconnections 36A and 37A are formed as so-called 50Ω lines.
The interconnections 36A to 38A, 39 and the pad portions 42 to 44, respectively, are made of a copper film or a gold film with a thickness of, for example, 35 microns. The predetermined regions of the high-frequency circuit interconnections 36A, 37A and the DC circuit interconnections 38A and 39 constitute microstrip lines and a bias circuit of λ/4. A second (lower) surface of the base material 45 is provided with a grounded ground layer 48A and land portions 49A to serve as external connection terminals.
A high-frequency active chip 33 is mounted on the high-frequency circuit board 32A of the above structure. An opening 46 is formed in the base material 45 at a position at which the active chip 33 is to be mounted. Also, a bottom open end of the opening 46 is closed by the ground layer 48A. Therefore, at a predetermined position of the high-frequency circuit board 32A at which the high-frequency active chip 33 is to be mounted, a recessed portion is defined by the opening 46 and the ground layer 48A.
The high-frequency active chip 33 is mounted in the opening 46 and is joined to the bottom ground layer 48A by means of gold-tin alloy or conductive adhesive agent. With the base material 45 provided with the ground layer 48A and the opening 46 and by mounting the high-frequency active chip 33 on the ground layer 48A in the opening 46, heat produced by the high-frequency active chip 33 can be dissipated in an efficient manner.
Also, the high-frequency active chip 33 and each interconnection 36A to 39A are electrically connected by wires 47. Since the high-frequency active chip 33 is positioned inside the opening 46, the wire bonding level of the high-frequency active chip 33 and the wire bonding level of the respective interconnections 36A to 39 are equal. Thus, wire bonding characteristic can be improved and the height of the wire loop can be reduced.
Also, a plurality of chip components 34 are mounted on a first (upper) surface of the high-frequency circuit board 32A. Each chip component 34 is joined to respective interconnection 36A to 38A and 39 or to the pad portions 42 to 44 by means of solder or conductive adhesive agent. The chip component 34 is a chip capacitor which together with the high-frequency circuit interconnections 36A and 37A form an input/output matching circuit. Although not shown in the diagrams referred to in the above description, a hybrid circuit (branch-line, coupler, rat-race, phase-reversion type hybrid, high-frequency filter, etc.) may be provided the first surface of the high-frequency circuit board 32A.
Also, the pad portions 42 to 44 are electrically connected to the ground layer 48A formed on the second surface of the base material 45 by means of via holes (not shown) formed through the base material 45. Further, a high-frequency input terminal 53, a high-frequency output terminal 54 and bias terminals 40 and 41 are formed at predetermined positions of the respective interconnections 36A to 38A. By means of the via holes 50 formed through the base material 45, the respective interconnections 40, 41, 53, and 54 are electrically connected to land portions 49A serving as external terminals.
As shown in FIG. 6, the land portions 49A are formed on the second surface of the base material 45 so as to be electrically separated from the ground layer 48A. The land portions 49A are electrically connected to the mounting board (not shown) when mounting the high-frequency module 30B. It is to be noted that, for ease of understanding, the via holes 50 are illustrated in FIG. 3, which is a cross-sectional diagram of FIG. 2 along line A—A, but practically, the via holes 50 do not appear in such a cross-sectional diagram.
Also, a resin package 35 is formed on the first surface of the high-frequency circuit board 32A. The resin package 35 is formed by, for example, transfer molding (hereinafter referred to as molding) and serves to protect the high-frequency active chip 33, the chip component 34, and the interconnections 36A to 38A and 39 that are formed on the first surface of the high-frequency circuit board 32A.
The high-frequency module 30A of the above structure uses a thin-film resin board of polyimide as the base material 45 of the high-frequency circuit board 32A. Since polyimide is less expensive compared to ceramics, the cost can be reduced in comparison to the high-frequency circuit board 2 with base material 15 of material such as ceramics (see FIG. 1).
Also, by using the base material 45 of polyimide, the thickness of the base material 45 can be reduced to about 25 to 75 μm. Thus, the width of the 50Ω lines can be reduced to about 50 to 150 μm, which in turn gives reduced area occupied by the 50Ω lines on the high-frequency circuit board 32A. Accordingly, it is possible to achieve reduced size and thickness of the high-frequency module 30A.
Also, although polyimide has a low relative permittivity of about 3.1, when the thin-film resin board of polyimide is used, the width of the 50Ω lines can be reduced by reducing its thickness. Therefore, it is no longer necessary to use a base material having high relative permittivity (e.g., ceramics, glass-ceramics, glass-epoxy etc.) which gives comparatively large thickness of the base material. This also serves to reduce the size and thickness of the high-frequency module 30A.
In general, as the frequency used becomes higher, the impedance of the interconnections 36A and 37A becomes greater. The less the thickness of the base material 45, the rate of increase of the impedance becomes smaller. Therefore, by using the polyimide having low relative permittivity as the base material 45, with a small thickness, low impedance can be maintained at a broad band frequency. Thus, an improved high-frequency circuit having high-frequency characteristics at broad band frequency can be achieved.
As has been described above, with the high-frequency module 30A shown in FIGS. 2 to 6, by using a thin-film resin board of polyimide as the base material 45, it is possible to provide a high-frequency circuit board 32A having an improved high-frequency characteristic at broad band frequency, low thermal resistivity and low cost. Thus, by incorporating the high-frequency module 30A of the present embodiment into a portable mobile terminal, a portable mobile terminal with reduced thickness can be achieved at a low cost.
Also, the base material 45 is a flexible substrate. Therefore, the high-frequency module 30A can be provided which can be provided at a low cost and which is not affected by the shape of the portable mobile terminal when the high-frequency module 30A is mounted on a portable mobile terminal.
However, the inventor has found that the base material 45 may be deformed during a molding step for forming the resin package 35. This will be described in detail in the following description.
As has been described above, the ground layer 48A and the land portions 49A are formed on the second surface of the high-frequency circuit board 32A. As shown in FIGS. 5 and 6, the land portions 49A are spaced apart at a predetermined interval (e.g., 600 μm).
Thus, the base material 45 is exposed at positions between neighboring land portions 49A. Also, the thickness of the ground layer 48A and the land portions 49A is, for example, 35 μm. Therefore, since the land portions 49A are raised and the portions where the base material 45 is exposed are recessed, the second surface of the high-frequency circuit board 32A becomes unleveled, or uneven.
FIG. 7 shows a resin molding step for forming the resin package 35. During the resin molding step, the high-frequency circuit board 32A is placed in mold 60A. The mold 60A comprises an upper mold 61A and a lower mold 62A. Cavities 63A and 64A are formed in the upper and lower molds 61A and 62A, respectively.
The cavity 63A of the upper mold 61A corresponds to the shape of the resin package 35 and the cavity 64A of the lower mold 62A has a planar shape. Therefore, when the high-frequency circuit board 32A is placed in the mold 60A, the land portions 49A touches the cavity 64A and gaps are formed at portions where the recessed portions 51 face the cavities 64A.
When resin 66 is injected into the cavities 63A, 64A from a gate 65 of the mold 64A, the base material 45 which is a thin-film resin substrate will be pressed by an injection pressure exerted by the resin 66. Since the gaps are formed at positions where the recessed portions 51 face the cavities 64A, the base material 45 deforms and sags at the gaps as indicated by dash-dot lines in FIG. 7.
Such deformation of the base material 45 may cause various problems. For example, the chip components 34 may fall off from the high-frequency circuit board 34A and the interconnections 36A, 37A, 38A, 39 may peel off from the base material 45. Further, the interconnections 36A, 37A, 38A, 39 may be disconnected due to stress applied thereto.